The American alligator (Alligator mississippiensis) is a large crocodylian reptile found throughout the southeastern United States. Individuals as long as 19 feet (6.5 m) have been reported, but ten or eleven feet (3.5 m) is more typical for large adults.

Alligators are semiaquatic, spending most of their time in the water but coming out on land to absorb heat periodically. They eat virtually anything they can swallow, but are not generally considered dangerous to humans - very few attacks have been reported. Other crocodylians, like the Nile crocodile of Africa and the Indopacific saltwater crocodile, are genuine maneaters.

There is another kind of alligator - the Chinese alligator (Alligator sinensis) - found in China. (To see a picture of a Chinese alligator, you can access the Crocodilian Photo Gallery.) Closely related to the alligators are the caimans, which are today found through much of Latin America. Together, the alligators and caimans form a group called Alligatoridae. The word "alligator" is sometimes used to refer to any member of Alligatoridae.

Like other crocodylians, female alligators lay their eggs on land. They guard their nests and, when the young alligators hatch, they protect the hatchlings. Baby alligators make a grunting sound to attract their mothers when they sense danger. Alligators are about nine inches (22.5 cm) long when they hatch. If fed well, young alligators grow quickly and can be 3 feet long (1 m) by the time they are three years old.

Like other reptiles, alligators are ectothermic ("cold-blooded") and cannot generate their own body heat. For this reason, they spend part of their time basking in the sun to absorb heat.

Why are alligators important?

In the Florida everglades, adult alligators excavate deep pits in the marshland. These are called alligator holes, and during the dry season they remain filled with water. They can be the only remaining standing water over a large area, and as such alligators help maintain this fragile wetland.

Alligators are also the top predator throughout much of their range. They fill an important role in keeping the populations of other wetland animals from growing out of control.

Alligators can also be economically important. Wild alligators were nearly hunted to extinction during this century because of demand for their skin, which can be used as exotic leather. Enlightened conservation policies during the 1970's and 1980's brought the alligator back from the brink of extinction, and alligator skins can now be provided to the leather industry from alligator farms and carefully regulated hunting.

Who are the Crocodylians?

Crocodylia includes living alligators, crocodiles, and gharials, and some of their closest extinct relatives. There are currently 23 living species within this group, but crocodylian diversity was much higher in the past.

The definition of Crocodylia has fluctuated in recent years. Older books may use the word "Crocodylia" to refer to living crocodylians and a large assemblage of extinct animals extending back to the Triassic period (roughly 245 million years ago). More recently, we have restricted the name Crocodylia to the last common ancestor of living alligators, crocodiles, and gharials, and all of its descendents. This excludes many of the extinct animals formerly considered to be crocodylians; these now belong to a larger group called Crocodyliformes. The oldest crocodylians are from the Late Cretaceous, roughly 80 million years ago.

You may have noticed that I spell "crocodylian" with a "y" rather than an "i", as in other books (i.e., "crocodilian"). The new spelling indicates that the more restricted defnition is being used.

What's the difference between an alligator and a crocodile?

This is one of the most common questions crocodile researchers face. There are 23 living species of crocodylian, and in fact not all are properly alligators or crocodiles - the Indian gharial (Gavialis gangeticus) is in a separate crocodylian lineage. Moreover, while some books use the word "crocodile" specifically for one of the crocodylids or "true crocodiles" (members of Crocodylus and Osteolaemus), others use "crocodile" to refer to any crocodylian, or even to a larger group of animals including Crocodylia and some extinct creatures not belonging to Crocodylia in the strictest sense. As such, the question "what's the difference between an alligator and a crocodile?" can be difficult to answer.

Systematists are scientists who study the diversity of living things and its organization. As a crocodile systematist, I regard a "crocodile," to be one of the the eleven living species of Crocodylus the single species of West African dwarf crocodile (Osteolaemus tetraspis), and those fossil taxa more closely related to them than to other crocodylians. The question is this: how are these different from alligators?

Many popular books state that alligators have blunt snouts, and crocodiles have narrow, pointed snouts. This sometimes works. Look at these two crocodylians:

Which one is the alligator, and which is the crocodile? The animal on the left is an American alligator (Alligator mississippiensis), and the animal on the right is a Cuban crocodile (Crocodylus rhombifer). In this case, the snout shape criterion works - the alligator does, indeed, have a broader, blunter snout.

But this doesn't always work!

For example, look at these two skulls:

If we can't always use snout shape, what can we do?

Which is the alligator? Which is the crocodile? In this case, the skull on the left is narrow, and that on the right is broad. Is the skull on the left a crocodile? In this case, no. The skull on the right, although broad, comes from a specimen of Morelet's crocodile (Crocodylus moreletii) from Mexico; the skull on the left is a rare form of common caiman (Caiman crocodilus apaporiensis), an alligatorid from Colombia. In this case, the snout shape criterion fails.

Perhaps the best single distinguishing characteristic between alligators and crocodiles is occlusion - the way the teeth come together when the jaws are closed. In alligators, the lower teeth occlude inside the upper teeth, and there is a complete overbite, as shown by the American alligator skull to the left.

But in crocodiles, the lower and upper teeth occlude along the same line, and the teeth interfinger. As a result, there is no overbite and the lower teeth are visible when the jaws are closed. This skull (Texas Memorial Museum of Science and History M-6801) from a Morelet's crocodile shows this well.

Did the alligator occlusion pattern evolve from the crocodile pattern, or was it the other way around? As it turns out, the common ancestor of alligators and crocodiles had an occlusal pattern "intermediate" between that of its living descendents.

This is the skull of an extinct animal closely related to living crocodylians (U.S. National Museum 22039). It comes from the middle Cretaceous (roughly 100 million years ago) of central Texas. It shows the occlusal pattern seen in the common ancestor of alligators and crocodiles. Note that the teeth toward the back of the jaw show an alligator-like overbite. But the fourth lower tooth, which is enlarged in all crocodylians, passes through a notch on the skull, as in crocodiles. Alligatorids lost this notch, and crocodylids lost the posterior overbite.

About the Species

This specimen (TMM M-6723) was made available to the University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Chris Brochu of The University of Iowa. Funding for image processing was provided by a National Science Foundation Digital Libraries Initiative grant to Dr. Timothy Rowe of The University of Texas at Austin.

This specimen was scanned by Matthew Colbert on 24 June 2005 along the coronal axis for a total of 680 slices. Each slice is 0.08 mm thick, with an interslice spacing of 0.08 mm and a field of reconstruction of 38.6 mm.

Neill, W.T. 1971. The Last of the Ruling Reptiles: Alligators, Crocodiles and their Kin. Columbia Univ. Press, New York.

Ross, C.A., and S. Garnett. (Eds.). 1989. Crocodiles and Alligators. Facts On File, Inc., New York. **perhaps the best book available on this subject.

More technical literature:

(please note: this is NOT an exhaustive list of the available literature. This is largely a list of references used in the completion of the computed tomography CD entitled "Alligator: Digital Atlas of the Skull." For a more complete listing of the literature, we recommend Mason Meers' on-line Bibliography of Crocodilian Biology.

Cott, Hugh B. 1961. Scientific results of an inquiry into the ecology and economic status of the Nile Crocodile (Crocodilus niloticus) in Uganda and Northern Rhodesia. Transactions of the Zoological Society of London (B), 29:211-357.

Currie, P.J. 1985. Cranial anatomy of Stenonychosaurus inequalis (Saurischia, Theropoda) and its bearing on the origin of birds. Canadian Journal of Earth Science, 22:1643-1658.

_______, and X.I. Zhao. 1993. A new troodontid (Dinosauria, Theropoda) braincase from the Dinosaur Park Formation (Campanian) of Alberta. Canadian Journal of Earth Science, 30:2231-2247.

de Beer, G.R. 1937. The Development of the Vertebrate Skull. Clarendon Press, Oxford.

Densmore, L.D., and P.S. White. 1991. The systematics and evolution of the Crocodilia as suggested by restriction endonuclease analysis of mitochondrial and nuclear ribosomal DNA. Copeia, 1991:602-615.

Erickson, B.R. 1972. Albertochampsa langstoni, gen. et sp. nov., a new alligator from the Cretaceous of Alberta. Scientific Publications of the Science Museum of Minnesota, New Series, 2:1-13.

Ferguson, M.W.J. 1981. The structure and development of the palate in Alligator mississippiensis. Achives for Oral Biology, 26:427-443.

_____. 1984. Craniofacial development in Alligator mississippiensis; pp. 223-273 in M.W.J. Ferguson (ed.), The Structure, Development and Evolution of Reptiles, Symposia of the Zoological Society of London 52, Academic Press, New York.

_____. 1985. Reproductive biology and embryology of the crocodilians; pp. 329-492 in C. Gans, F. Billett, and P.F.A. Maderson (ed.), Biology of the Reptilia, Vol. 14, John Wiley and Sons, New York.

Hall, P.M., and K.M. Portier. 1994. Cranial morphometry of New Guinea crocodiles (Crocodylus novaeguineae): ontogenetic variation in relative growth of the skull and an assessment of its utility as a predictor of the sex and size of individuals. Herpetological Monographs, 8:203-225.

Klembara, J. 1991. The cranial anatomy of early ontogenetic stages of Alligator mississippiensis (Daudin, 1802) and the significance of some of its cranial structures for the evolution of tetrapods. Paleontographica, 215:103-171.

Nilsson, T. 1944. On the morphology of the lower jaw of Stegocephalia with special reference to Eotriassic stegocephalians from Spitsbergen. Kungl. Svenska Vetenskapsakademiens Handlingar, 3 Ser., 21:1-70.

Owen, R. 1850a. On the communications between the cavity of the tympanum and the palate in the Crocodilia (gavials, alligator and crocodiles). Philosophical Transactions of the Royal Society of London, 27:521-527 + 3 plates.

_____. 1850b. Monograph on the Fossil Reptilia of the London Clay, and of the Bracklesham and Other Tertiary Beds, Part II: Crocodilia (Crocodilus, etc.). Paleontographical Society, London, 50 pp. + 11 plates.

Parker, W.K. 1883. On the structure and development of the skull in the Crocodilia. Transactions of the Zoological Society of London, 11:263-311.

Patterson, B. 1931. Occurrence of the alligatoroid genus Allognathosuchus in the lower Oligocene. Field Museum of Natural History Geological Series, 6:223-226

Walker, A.D. 1990. A revision of Sphenosuchus acutus Haughton, a crocodylomorph reptile from the Elliot Formation (Late Triassic or Early Jurassic) of South Africa. Philosophical Transactions of the Royal Society of London B, 330:1-120.

Witmer, L.M. 1995. Homology of facial structures in extant archosaurs (birds and crocodilians), with special reference to paranasal pneumaticity and nasal conchae. Journal of Morphology, 225:269-327.

_____. 1997. The evolution of the antorbital cavity of archosaurs: A study in soft-tissue reconstruction in the fossil record with an analysis of the function of pneumaticity. Society of Vertebrate Paleontology Memoir 3:73.

Wu, X.C., D.B. Brinkman, and A.P. Russell, 1996. A new alligator from the Upper Cretaceous of Canada and the relationships of early eusuchians. Palaeontology, 39:351-375.